The present invention relates to current source biasing circuitry for various electronic devices, including but not limited to electronic circuitry in mobile telecommunication terminals. More specifically, the present invention relates to a current mirror circuit having an input portion with a first transistor, an output portion with a second transistor, and a control portion between the input portion and the output portion for controlling the second transistor to generate an output current which is a function of a reference current established by the first transistor.
Generally, biasing serves to establish a selected operating range in the voltage-current characteristics of non-linear devices such as transistors and diodes, so that gross non-linearities of the non-linear devices will be avoided and so that, therefore, the non-linear devices will behave like or resemble linear devices.
Current source biasing serves to provide a constant DC current source for the circuit to be biased. Current mirrors provide current source biasing which is substantially independent of device temperatures and device parameters, such as the forward DC current gain xcex2f of a bipolar junction transistor (BJT), or the conductance parameter (K) and the threshold voltage (VTR) for a field-effect transistor (FET). Current mirrors are described in detail in sections 7.8-7.10, pp 302-314 of xe2x80x9cMicroelectronic Circuits and Devicesxe2x80x9d, by Mark N Horenstein, Prentice-Hall, Inc., 1990, ISBN 0-13-584673-0, incorporated herewith by reference.
As is shown in FIG. 2, a prior art current mirror uses a pair of first and second three-terminal devices T1 and T2, coupled xe2x80x9cback-to-backxe2x80x9d with their input ports connected in parallel. T1 and T2 may be of arbitrary type but are illustrated as bipolar junction transistors in FIG. 2. T1 and T2 are connected to ground through their emitter terminals and first and second resistors R1, R2, respectively. The transistor T1 and resistor R1 form an input portion 152, which receives an input current iin and establishes a reference current iref. Correspondingly, the transistor T2 and resistor R2 form an output portion 154, which serves to supply an output current iout to a circuit 160 to be biased. To this end, a control portion 156 is formed between the input portion 152 and the output portion 154.
In the prior art solution according to FIG. 2, the control portion 156 simply consists of a connection between the collector terminal of transistor T1 and the base terminal of transistor T2, having a drawback in that some current it will be stolen from the input current iin to control the transistors T1 and T2.
In another prior art solution, shown in FIG. 3, the above is remedied by introducing a third transistor T3 into the circuit. Transistor T3 is biased trough Vcc and will enable the current mirror to operate without stealing current from iin. The transistor T3 may be, for instance, a bipolar junction transistor or an NMOS field-effect transistor. If T3 is a bipolar junction transistor, the stolen current it will be close to 0, and if T3 is an NMOS, it will equal to 0 (wherein iref will be equal to iin).
However, the present inventors have identified a remaining problem with the prior art solution according to FIG. 3. At node 164 in the input portion 152, when looking both towards transistor T1 and towards input node 162, the impedances seen will be very high, particularly when BICMOS circuits are used. Therefore, any noise introduced at node 164 will go entirely across the control portion 156 to transistor T2 in the output portion 154, thereby corrupting the output signal at node 166. Corruptions in the output signal mean that the following circuit 160 will not be biased with a perfect DC current source, and if this circuit is designed to rely on a perfect DC current source, the operation thereof may be jeopardized.
In view of the above, an objective of the invention is to solve or at least reduce the problems discussed above and to provide an improved current mirror technique compared to the prior art.
Generally, the above objective is achieved by a current mirror circuit according to the attached independent claim.
Thus, a first embodiment of the invention concerns a current mirror circuit comprising: an input portion including a first transistor, wherein the first transistor is adapted to establish a reference current; an output portion including a second transistor; and a control portion between the input portion and the output portion, the control portion including a third transistor coupled for controlling the second transistor to generate an output current which is a function of the reference current while inhibiting current leakage from the input portion to the output portion.
The objective of the invention has been achieved, in the first embodiment, by introducing a lowpass filter in the control portion prior to the third transistor. The lowpass filter will filter the signal that controls the second transistor, thereby allowing any noise from the input portion to be filtered out before it is amplified by the second transistor to generate the output current. Additionally, including the lowpass filter within the current mirror circuit has an advantage compared to external filtration outside/after the current mirror, since additional power would be consumed in the latter case.
In a second, more sophisticated embodiment, the lowpass filter is an RC filter which, as its resistive part, comprises a fourth transistor which is biased by a feedback loop into a state of high impedance. The state of high impedance may be the resistive (also known as triode or ohmic) region of operation for the fourth transistor, which may be a PMOS field-effect transistor.
Using a PMOS field-effect transistor instead of a resistor in an RC filter is advantageous for the following reasons. Due to the very high impedances in the input portion, a resistor would need to be very large (in the order of 1 Gxcexa9) to establish a proper low filtering frequency. However, resistors with such high impedance are very hard to build. Alternatively, the capacitor in the RC filter could be made very large, but that would be very costly because of the physical size of a large capacitorxe2x80x94silicon chip space is very expensive.
Moreover, by selecting a PMOS transistor, the gate-source voltage for the third transistor can be chosen so that it is sufficient to bias the fourth transistor, wherein no other components are necessary. By biasing the fourth transistor in a feedback loop, the ohmic characteristics thereof will be stabilized. Consequently, this embodiment of the invention overcomes a known problem in the technical fieldxe2x80x94because of the inherently irregular (non-linear) ohmic characteristics of a PMOS transistor it is difficult to include a PMOS transistor in a lowpass filter of RC type.
In either of the embodiments the third transistor may be an NMOS field-effect transistor or another type of field-effect transistor. The first and second transistors may be bipolar junction transistors, but essentially any other three-terminal devices would also do.
In a third embodiment, the lowpass filter comprises a plurality of cascaded RC filters, each of which has a PMOS field-effect transistor as its resistive part. This arrangement allows efficient noise filtering at a low component cost.
Generally, the current mirror circuit according to the invention may be included in any application using current mirrors. In one embodiment the current mirror circuit according to the invention is included in a station for a mobile telecommunications network. The station may be a mobile terminal.
Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to xe2x80x9ca/an/the [element, device, component, etc]xe2x80x9d is to be interpreted openly as referring to at least one instance of said element, device, component, etc., unless explicitly stated otherwise. As used herein, the term xe2x80x9ctransistorxe2x80x9d embraces all electronic three-terminal devices having a constant-current region, unless explicitly stated otherwise.